1. Field of the Invention
The present invention relates to a probe microscope for observing a fine surface configuration of a sample by use of a probe, and more particularly to a force microscope such as an atomic force microscope or a magnetic force microscope which utilizes an inter-atomic force or magnetic force acting between a sample and a probe.
2. Description of the Related Art
One of apparatuses capable of observing a surface configuration of an insulating sample in the atomic order is an atomic force microscope (AFM) which is described in detail, for example, in "G. Binnig, C. F. Quate, "Atomic Force Microscope", Physical Review Letters, Vol. 56, 930 (1986). When a pointed probe is approached towards the surface of a sample (hereinafter referred to as "sample surface"), an attractive force due to van der Waals mutual effect is generated between an atom at the tip of the probe (hereinafter referred to as "probe tip atom") and an atom on the sample surface (hereinafter "sample surface atom"). If the probe is further approached to the sample surface and the distance between the probe tip atom and the sample surface atom is decreased substantially to a bond distance, a repulsive force based on the Pauli exclusion principle acts therebetween. The attractive force and repulsive force are generally called "atomic force", and the magnitude of the atomic force is very is very low, e.g. about 10.sup.-19 to 10.sup.-12 [N]. The atomic force microscope observes the sample surface configuration by utilizing the atomic force. The atomic force microscope will now be described in brief. In this microscope, a probe is attached to a soft cantilever. The probe is approached to the sample until an atomic force acts between the probe tip atom and sample surface atom and the cantilever is bent to some extent. In this state, the probe is moved along the sample surface. When the distance between the probe and sample varies in accordance with the sample surface configuration, the amount of displacement of the cantilever also varies. The distance between the probe and the sample is controlled by using a fine movement element such as a piezoelectric element, so as to restore the amount of displacement of the cantilever to an initial value. While the sample surface is raster-scanned, the control of the distance is carried out so that the locus of the probe tip describes a curved plane representing the sample surface configuration. Thus, an image of the sample surface configuration can be obtained on the basis of positional data of the probe tip.
On the other hand, the magnetic force microscope (MFM) is designed to observe the surface configuration of a magnetic material (sample). The structure of the MFM is identical to that of the above-described AFM except that magnetic force is utilized in place of atomic force. In other words, the basic structure of the MFM differs from that of the AFM in that the probe of the MFM is formed of a magnetic material. The method of observation is common between the MFM and the AFM. The probe is moved along the sample surface with a magnetic force kept constant between the probe and a magnetic particle of the sample, thereby obtaining an image of the sample surface configuration on the basis of positional data of the probe tip.
In the AFM and MFM, the sample surface configuration is measured by moving the probe relative to the sample surface in x- and y-directions, with the distance between the probe and the sample kept constant. The probe is (servo) controlled by moving the probe in a z-direction perpendicular to the sample surface by using, for example, a fine movement element such as a piezoelectric element.
FIG. 10 shows a general relationship between the force and the distance between the probe tip atom and the sample surface atom in the case where the probe approaches the field in which the atomic force or physical/chemical adsorption force due to atoms/molecules near the sample surface is generated. For example, when the distance between the probe and the sample is servo-controlled to a constant value so as to keep the attractive force at point A (in FIG. 10) constant, the probe does not collide with oxide film or dust on the sample, even if it exists, as far as the distance between the probe and oxide film (or dust) on the sample is exactly servo-controlled.
Both in the AFM and MFM, the distance between the probe and the sample is servo-controlled and kept constant while the sample surface is scanned by the probe (in x- and y-directions). The probe is moved in the x- and y-directions while it is moved in the z-direction so as to keep the distance Az between the probe and sample constant. In this case, if a steep surface portion (upwardly inclined portion) and the probe moves towards it, the probe would collide with it. The reason for this is that the gradient .theta..sub.T of the sample near the probe tip is gentle and the distance .DELTA.X.sub.T over which the probe moves while rising in response to the servo control is greater than the distance .DELTA.X.sub.S between the side surface of the probe and the steep surface of the sample. In particular, where the gradient .theta..sub.T is zero or less, the collision of the probe with the steep surface is inevitable. In the case where there is a downwardly inclined steep surface portion, the same problem occurs.